1. Field of the Invention
This invention relates to a method for the manufacture of a lead frame having an inner lead part thereof coated with aluminum.
2. Description of the Prior Art
Ceramic packaging is one of the packaging systems useful for semiconductor integrated circuit (IC) devices. This system achieves the necessary packaging by fastening a lead frame of Fe-Ni alloy to a ceramic substrate possessing a cavity for accommodating an IC device with glass of a low melting point, securing the IC device on the bottom of the cavity in the substrate with solder or glass, connecting an electrode on the IC device to the inner lead part of the lead frame with a bonding wire, setting a ceramic lid in place on the substrate, and sealing the resultant package with glass of a low melting point. Since the package formed by this system excels in heat radiation, the system finds popular utility in applications to IC devices of relatively large output and IC devices which have low heat radiation due to high-speed operation. The following consideration is accorded to this system for the preservation of its reliability.
Generally, the electrodes on the IC devices are made of aluminum (Al). When gold wires are bonded to these Al electrodes, an Au-Al intermetallic compound called "plague phenomenon" is formed at the sites of bonding when the sites are exposed to heat. Since this substance is very brittle, the bonding wires may separate. In the package of this system, therefore, it is necessary to use Al bonding wires. Further, the inner lead part is generally plated with gold or silver to facilitate bonding. When Al wires are connected to the coating of gold or silver, the phenomenon mentioned above occurs at the sites of bonding. As the metal for plating the inner lead part, therefore, adoption of Al is normally expected.
Incidentally, no technique has yet been established for electroplating a given substrate with Al in a wet state. The conventional Al-coating lead frame, therefore, has been manufactured by press punching a strip of clad material having a Al tape applied fast thereon along the central zone thereof.
In the clad material produced in this manner, however, the outer lead must be formed in the portion not covered with the Al tape, i.e., exclusively in the two directions along the opposite edges of the Al strip. As a consequence of the recent advances in the degree of integration of IC devices, the number of electrodes formed on IC devices is increasing. For this reason, the necessity for a lead frame having outer leads extended in the four directions (hereinafter referred to as "quard type lead frame") has been receiving growing recognition. When the lead frame is in such a shape as described above, Al is required to be deposited in the form of spots exclusively in the inner lead part. The lead frame of the foregoing description cannot be obtained from the aforementioned material for the clad and the production of a clad material possessing an Al coating in the form spots is difficult to obtain. For this reason, adoption of means of physical deposition such as the vacuum evaporation method, the ion plating method, or the spattering method have been contemplated. Some of these methods have found utility in actual applications. Unfortunately, none of these methods has been found fully satisfactory.
For example, the vacuum evaporation method has the disadvantage that the Al coating produced is destitute of tightness of adhesion and the Al coating is liable to recrystallize and, due to the heat which is applied during the aforementioned process of packaging, the metal forming the web of the lead frame is diffused into the Al coating to the extent the reflectance of the Al coating is degraded. The recent wire bonder is so advanced as to detect the Al-covered surface based on differences in reflectance and automatic bonding. The degradation of reflectance, therefore, is detrimental to automatic bonding. The ion plating method has the advantage that the coating produced thereby enjoys sufficient tightness of adhesion because it effects deposition of the coating generally by using argon gas at 10.sup.-2 to 10.sup.-3 Torr, ionizing vaporized atoms under glow discharge, and causing the resultant ions to collide against a substrate kept under a potential of -1 to -5 KV. In the meantime, this method has the disadvantage that this coating has a high hardness. This high level of hardness of the coating may be ascribed to permeation of argon gas in the Al coating. The spattering method utilizes glow discharge of argon gas and, in this respect, equals the ion plating method. The Al coating produced by this method has high hardness. The hardness of the Al coating affects the bonding property of the Al wire. Under the fixed bonding conditions, the fraction of defective bonding decreases in proportion as the hardness decreases. Where a fixed fraction of defective bonding is tolerated, the speed of bonding is increased in proportion to a decrease in hardness.